Coriolis effect mass flow rate meters are well-known. One such meter that has gained widespread commercial acceptance is shown in U.S. Reissue Patent 31,450 to James E. Smith of Nov. 29, 1983. As taught by Smith, the flow of a material through an oscillating conduit produces Coriolis forces which are perpendicular to both the velocity of the mass moving through the conduit and the angular velocity vector of the oscillation of the conduit. The magnitude of the generated Coriolis forces is related to the material mass flow rate as a function of the angular velocity of the mass flowing through the conduit.
Coriolis effect flow meters typically use one or two flow tubes to direct the measured material flow from a pipe, through the meter tubes and then back to the pipe. These tubes may be either straight or curved, or irregular shaped, and they may be mounted in the flow line or attached to a substantially rigid support. The tubes are normally vibrated by an electromagnetic drive at the natural frequency of the tube structure including the measured material. The Coriolis forces resulting from the mass of the material flow through the vibrating tubes causes a displacement of portions of the tubes. The displacement is measured at points on the tubes by position or velocity sensors. The time differential .DELTA.t between the movements of the tube elements at spaced apart locations is used for a determination of information including the mass flow rate of the measured material.
One step in measuring the generated Coriolis forces is to track the relative movement of different portions of the legs of meters having U-shaped tubes. This is typically done by attaching two electromagnetic velocity sensors each comprising a magnet and a pickup coil in opposing relative positions on the side legs of the flow conduit or conduits as described in U.S. Pat. No. 4,422,338 entitled, "Method and Apparatus for Mass Flow Measurements" and issued Dec. 27, 1983, to James E. Smith. This is also shown in U.S. Pat. No. 4,491,025 of Jan. 1, 1985, to James E. Smith and Donald R. Cage. In a parallel dual tube design as disclosed in the patent to Smith and Cage, a sensing coil is attached to one of the two flow conduits. A cooperating magnet mounted to the other flow conduit is positioned coaxially within the sensing coil. As the tube is vibrated by the drive coil, the sensing coil produces a signal which is representative of the movement of the conduit leg. By this means, a complete velocity profile is generated for each leg. The signals generated by the two sensors are applied to signal processing circuitry which produces an output representing the desired information for the flowing material (such as, for example, the mass flow rate, the density, etc).
Although the currently available Coriolis effect meters (including those disclosed in the above-identified patents) operate satisfactorily and produce excellent results under most conditions, there are certain circumstances in which their performance is not wholly satisfactory. For example, since they use electromagnetic devices as sensors, these devices can be affected by external magnetic fields. Under such circumstances their output data may be subject to error.
Electromagnetic sensors are also complex to manufacture due to the small gauge of the wire used in the sensing coils and due to the required resin coating and curing process for protecting the coils. Despite the resin coating, the sometimes harsh operating environment for these meters can cause the sensor coils to fail.
Another disadvantage is that the coils are inductive devices which store energy that can generate arcing. This is a problem if the meter is used in an explosive atmosphere.
Also, meters using these sensors are often used to measure the flow of material at high temperatures. These temperatures are often at such a level that the magnetic coils of these sensors have a high failure rate or else become unstable and generate output data that is unreliable.
In view of the above, it can be seen that there currently exists a need for Coriolis effect meters having sensors which are immune or resistant to harsh environmental conditions (such as, for example, strong electromagnetic fields or high temperatures). It can further be seen that there exists a need for sensors which are more economical to manufacture, are more reliable, are immune to explosion hazards, and are both mechanically and thermally rugged.